A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissues. Some therapies include the delivery of electrical signals, e.g., stimulation, to such organs or tissues. Some medical devices may employ one or more elongated electrical leads carrying electrodes for the delivery of therapeutic electrical signals to such organs or tissues, electrodes for sensing intrinsic electrical signals within the patient, which may be generated by such organs or tissue, and/or other sensors for sensing physiological parameters of a patient.
Medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of therapeutic electrical signals or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to a medical device housing, which may contain circuitry, such as signal generation and/or sensing circuitry. In some cases, the medical leads and the medical device housing are implantable within the patient. Medical devices with a housing configured for implantation within the patient may be referred to as implantable medical devices.
Implantable cardiac pacemakers or cardioverter-defibrillators, for example, provide therapeutic electrical signals to the heart via electrodes carried by one or more implantable medical leads. The therapeutic electrical signals may include pulses or shocks for pacing, cardioversion, or defibrillation. In some cases, a medical device may sense intrinsic depolarizations of the heart, and control delivery of therapeutic signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate therapeutic electrical signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing stimulation to the heart of the patient upon detecting bradycardia or tachycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
If noise, such as noise due to electromagnetic interference (EMI), is present on a signal used to sense depolarizations of the heart, a medical device may incorrectly detect abnormal cardiac rhythms. More particularly, rapid signal fluctuations due to EMI or other noise may be interpreted by a medical device as a plurality of sensed cardiac depolarizations, and result in the medical device inappropriately detecting a cardiac arrhythmia. The rate of sensed events when noise is present may be similar to or greater than that for detection of a tachyarrhythmia, and the medical device may detect a tachyarrhythmia based on the noise.